Greener power for off-grid sites
Combined heat and power units have been used by large businesses for many years, using internal combustion engines to provide electricity via a generator, and heat from the engine cooling system and exhaust.
On small premises, turning on a pump or kettle can increase the electrical load several fold in a second. Attempts to cope with the now unmatched electricity and heat results in the system running inefficiently.
According to the consortium, the solution is a generator that runs constantly at high efficiency, coupled to an electrical energy store.
“The challenge, was to design a system that could simultaneously satisfy the more predictable needs for heating and hot water, as well as the wildly varying demand for electricity in a small dwelling,” said Professor Tony Roskilly of Newcastle. “Our solution was to incorporate electrical storage into the system, both batteries and supercapacitors, combined with system control.”
Excess heat is stored in hot water tanks for heating and hot water.
Refrigeration and air conditioning operates via an absorption chiller running off heat.
Leeds University, part of the consortium alongside Newcastle, the University of Ulster, and three Chinese universities, has also patented a cryogenic energy store.
This combination has been dubbed ‘trigeneration’.
“We wanted to avoid running the trigeneration system using biodiesel or other highly-processed fuels from raw materials,” says Professor Roskilly. “So instead, we developed a system for using the oils obtained from pressing crop seeds, like those from jatropha and croton. These can grow in harsh environments and on poor-quality land and so could be well-suited to providing fuel in developing countries, as cultivating them would not adversely affect food production.”
£1.13m of funding came from the Engineering and Physical Sciences Research Council (EPSRC) through the RCUK Energy Programme.
Before designing the trigenerator, the team logged the minute-by-minute energy use in households. “Previous studies have lost important detail by averaging demand over much longer timescales,” said the EPSRC. “In a typical UK house, heating demand is largely stable when hot water and space heating is required. Electricity consumption can hover around a hundred watts most of the day, but reach peaks of 7kW in a matter of seconds, and for just a minute or two.”
In a follow-up study funded by the EPSRC, the Department for International Development and the Department of Energy & Climate Change, Roskilly is exploring how the trigeneration can be used on small farms in the developing world to refrigerate and process food crops, to reduce post-harvest losses.
The Newcastle team is examining long-term performance running on raw plant oils, and is discussing commercialising the design with potential manufacturers.
A domestic-scale tri-generation system would be rated between 6kW and 9kW of electricity, equivalent to the combined consumption of: lights, TV, fridge freezer, kettle, microwave, vacuum cleaner, washing machine, and dishwasher.